This invention relates generally to semiconductor structures and manufacturing methods and more particularly to bipolar transistor structures and manufacturing methods.
As is known in the art, one technique used in the formation of bipolar integrated circuit transistors is to provide an n-type conductivity epitaxial layer on a p-type conductivity silicon substrate. The transistors are formed in the epitaxial layer and are electrically isolated from each other by silicon dioxide isolation regions which are formed in the epitaxial layer around the active devices. The epitaxial layer provides the collector region for the transistor. A p-type conductivity region is generally diffused into the epitaxial layer to form the base region for the transistor. The emitter region is generally formed by diffusing an n-type conductivity region into the p-type conductivity base region.
In order to reduce the amount of surface area required for the transistor a portion of the p-type conductivity base region is bounded by the silicon dioxide isolation region. In such a transistor the emitter region is formed entirely within the base region since extension of the emitter region to the silicon dioxide isolation region may cause the emitter region to become electrically short-circuited to the collector region. In particular, if a portion of the emitter region in such transistor extended to the silicon dioxide isolation region, charges in the silicon dioxide may cause an inversion of the more lightly doped, bottom portion of the p-type conductivity diffused base region adjacent the silicon dioxide isolation region. This inversion results in an n-type conductivity channel being formed through such lightly doped portion of the base region between the underlying n-type conductivity epitaxial layer (which forms the collector region) and the n-type conductivity emitter region. To prevent this channel, and hence short circuit, from being formed between the emitter and collector the emitter region is generally formed totally within the upper, more heavily doped, portion of the base region. This upper, more heavily doped, portion of the base region, while extending to the silicon dioxide isolating region, is disposed between the emitter region and the isolation region to inhibit inversion caused by charges in the silicon dioxide isolation region and thereby prevent an n-type conductivity channel, a short circuit, from being created between the epitaxial layer and the emitter region. Since the emitter region is formed totally within the base region, however, an increased area is required to form the transistor, thereby reducing the number of active devices which may be formed in a silicon chip.